The present invention relates to methods for fabricating semiconductor devices such as light-emitting diodes or semiconductor lasers.
Group III-V nitride semiconductors (InGaAlN) containing gallium nitride (GaN) as a main component have wide bandgaps, and thus are applicable to light-emitting devices such as visible-light-emitting diodes which emit blue or green light or short-wavelength semiconductor lasers. In particular, light-emitting diodes have been put into practical use for large-screen displays or traffic lights. White-light-emitting diodes which emit light by exciting fluorescent materials are expected to replace currently-used lighting units.
Semiconductor lasers are also expected to enter mass volume production in the near future for implementation of high-density high-capacity optical disk apparatus using violet-light emitting lasers.
In the past, crystal growth of a nitride semiconductor has been difficult as other wide-bandgap semiconductors. However, techniques for crystal growth, which are metal organic chemical vapor deposition (MOCVD) processes in most cases, have been greatly developed, so that the above-mentioned diodes have been put into practical use.
A substrate for epitaxial growth of the nitride semiconductor is difficult to form using gallium nitride, and crystal growth on a substrate made of the same material as that of an epitaxial growth layer, as used for crystal growth of a semiconductor crystal such as silicon (Si) or gallium arsenide (GaAs), is not easily performed. Therefore, hetero-epitaxial growth performed on a substrate made of a material other than the nitride semiconductor is generally used.
Until now, sapphire has been most widely used for the hetero-epitaxial growth of a nitride semiconductor and exhibits the most excellent device characteristics. However, since sapphire has an insulating property, if a light-emitting diode including a pn junction, for example, is formed on a sapphire substrate, a substrate-side surface of a nitride semiconductor layer constituting the pn junction needs to be exposed by etching so that p- and n-side electrodes are formed on an epitaxial-layer-side surface of the substrate. As a result, the chip area increases as well as the series resistance increases.
In addition, sapphire has low heat conductivity. Thus, if a semiconductor laser, for example, is formed on a sapphire substrate, heat radiation from the laser deteriorates, thus shortening the lifetime of the laser.
One of the methods for solving the problems is using a conductive substrate superior to a sapphire substrate in heat radiation characteristic, as a substrate on which a nitride semiconductor is grown, instead of a sapphire substrate. Crystal growths using a silicon (Si) or gallium arsenide (GaAs) substrate have been vigorously researched and developed up to now, but no material superior to sapphire in crystallinity has been achieved.
In view of this, a so-called transfer method with which an epitaxial semiconductor layer of nitride grown on a sapphire substrate and having excellent crystallinity is separated from the sapphire substrate and then is transferred onto a substrate (i.e., different-type substrate) as an alternative to a sapphire substrate has been studied.
To separate the sapphire substrate from the epitaxial semiconductor layer, it is possible to remove the sapphire substrate by polishing. However, there also occur other problems that it is difficult to control the polishing of the sapphire substrate and that the sapphire substrate on which the nitride semiconductor has been grown is warped to be in a convex shape because of the difference in thermal expansion coefficient between nitride semiconductor and sapphire. To eliminate these problems, a laser lift-off technique, i.e., a technique for separating a sapphire substrate, has been developed (in Japanese Journal of Applied Physics, Vol. 38 (1999) pp. L217-L219 by M. K. Kelly et al. and Applied Physics Letters, Vol. 72 (1998) pp599-601 by W. S. Wong et al.). Specifically, after the nitride semiconductor layer has been grown on the sapphire substrate, the nitride semiconductor layer and the sapphire substrate are irradiated with a KrF excimer laser light beam with a wavelength of 248 nm or a YAG laser third-harmonic light beam with a wavelength of 355 nm. Each of the laser light beams is a short-pulse laser light beam with very high optical power and passes through the sapphire substrate to be absorbed only in the nitride semiconductor layer. This light absorption causes part of the nitride semiconductor layer near the interface between the nitride semiconductor layer and the substrate to generate heat locally, so that decomposition by heat occurs if the output of the laser light beam is sufficiently high. As a result, a decomposition layer containing metal gallium (Ga) created by the decomposition by heat is formed at the interface between the nitride semiconductor layer and the sapphire substrate. Accordingly, if the decomposition layer is removed either by heating to a temperature almost greater than the melting point of Ga or by using an acid solution, the nitride semiconductor layer is separated from the sapphire substrate.
However, since a nitride semiconductor layer for use in a light-emitting device has a thickness of as small as about 5 μm to 10 μm, handling of the nitride semiconductor layer (wafer) from which the sapphire substrate has been separated is extremely difficult.
Thus, to ease the handling of the wafer from which the substrate has been separated, shown was a first method with which a different-type substrate of, for example, silicon is bonded to a surface of the nitride semiconductor layer opposite to the sapphire substrate, and then the sapphire substrate is separated by a laser lift-off process so that the nitride semiconductor layer is transferred onto the different-type substrate (in Applied Physics Letters, Vol. 77 (2000) pp. 2822-2824 by W. S. Wong et al.).
As a second method, the following method was disclosed. A supporting substrate of, for example, silicon, is bonded to a surface of the nitride semiconductor layer opposite to the sapphire substrate via an organic adhesive interposed therebetween, and then the sapphire substrate is separated from the nitride semiconductor layer by a laser lift-off process. Subsequently, after the nitride semiconductor layer has been bonded to a different-type substrate of, for example, copper (Cu), the organic adhesive is removed, so that the supporting substrate is separated from the nitride semiconductor layer, thereby transferring the nitride semiconductor layer onto the different-type substrate (in Compound Semiconductor Vol. 7, (2001) pp47-49 by W. S. Wong et al.).
With the first or second method, the sapphire substrate is removed and the nitride semiconductor layer is transferred onto a conductive different-type substrate replacing the sapphire substrate. In this way, the p- and n-side electrodes can be formed so as to oppose to each other with the different-type substrate sandwiched therebetween. As a result, the chip size and the series resistance can be reduced. In addition, the heat radiation characteristic improves, thus allowing improvement in performance of the device.
However, with the first known method, while the temperature is reduced to room temperature after the epitaxial growth of the nitride semiconductor layer on the sapphire substrate, the substrate is warped to be in a convex shape because of the thermal expansion coefficient between sapphire and nitride semiconductor. Therefore, there occurs a problem that it is extremely difficult to bond the different-type substrate of silicon to the nitride semiconductor layer uniformly in a relatively large area.
The second known method also has a problem that it is difficult to completely remove the organic adhesive used for bonding the supporting substrate to the nitride semiconductor layer.